The Ubuntu Wiki has a very thorough guide that is well illustrated and covers many possible scenarios.

The essence of that page is that you first need a Launchpad account, then press Alt+F2 and enter ubuntu-bug packagename. In 11.04 and later you can enter ubuntu-bug -w and click on the affected window.

There are also methods described on that wiki page telling you how to:

• file bugs against running programs
• against no particular package (and how to decide which package)
• and from computers with no internet access.

There is a quick tutorial video that explains better.

And some more information on this site about how to gather information for a bug report:

• I've got Internal Error whilst running a Ubuntu Beta. How to gather information and find out what it is?

The most general, and comprehensive set of instructions can be found at this community-maintained help page. This answer essentially, consolidates that page, and hence it is advised that you read it, and if you are hunting bugs, it is required that you read it.

Short version:

• Know the package at fault? go type ubuntu-bug <packagename> in a terminal.
• Have a window before you thats buggy? go type ubuntu-bug -w in a terminal, and click on the window.
• Have the pid? go type ubuntu-bug <pid> in a terminal.
  You may also use Run dialog (Alt+F2) instead of terminal.

Long version:

Step one: Preliminaries

• Create an account at Launchpad, if you don't already have one.
  It is advised that you read Launchpad Help on creating a new account.
  Direct Login/signup link here
• Determine if it is really a bug.
• Make sure the bug hasn't been already reported by searching Existing Bugs.

  This is important to not trouble the volunteer Bug Squad, who help in resolving bugs. If you found an existing bug, you can answer 'Does this bug affect you' with 'This bug affects me' and add any additional information as comments.

Step two: Determine the package at fault

If you know which package is at fault, you can run ubuntu-bug <packagename> to initiate the bug reporting process. you may now proceed to the next step.
Detailed instructions on finding the right package are given here And if those detailed instructions didn't help, you have a last resort.

As a last Resort:

You can type ubuntu-bug in a terminal, or Run window, and answer few questions to gather some information on the bug.

If all else fails, and you were unable to determine the right package, you may ask for help in IRC channels, or go ahead and report a bug against no particular package. Kindly note that this adds extra effort on our small group of volunteers, and you are requested to use it, as the title reads, only as a last resort. Also note that this may delay the process of resolution for your bug, and thats another reason to put in all the effort you can, to find the right package.

You may file a bug against no particular package by going here.

Step three: Submitting the bug report

(quoted verbatim from ReportingBugs for completeness.)

After executing any of the above commands, Apport (Ubuntu bug-reporter) will collect necessary data. A window will then pop up, asking you if you want to report the bug. Click "Send Report" if you wish to proceed, or click "Content of the report" if you want to review the information Apport collected.

Apport will then upload the problem information to Launchpad, and a new browser window will then open to inform you that the bug report is being processed. After the bug report data has been processed, a new page will open that will ask you for the bug report's title. The bug title will appear in all bug listings so make sure it represents the bug well. When you're done, click "Next". A search will then occur based on the title you gave to the bug report, and will show potentially similar ones. If one of these seems to be the exact bug you're reporting, click its title, then "Yes, this is the bug I'm trying to report". If not, click "No, I need to report a new bug". Launchpad will then ask you for further information. It's important that you specify three things:

• What you expected to happen
• What actually happened
• If possible, a minimal series of steps necessary to make it happen, where step 1 is "start the program"

Fill in the description field with as much information as you can, it is better to have too much information in the description than not enough.

At then bottom of the page, there are some extra options you can use to make your bug report more complete:

• This bug is a security vulnerability: Please check this only if your bug report describes a behaviour that could be exploited to crack your system. (TODO: better wording?)

• Tags: You can add here tags that pertain to your bug report. The predefined values should be left alone.

• Include an attachment: Using this option, you can add supporting attachments to explain or help others reproduce the bug. This might include a screenshot, a video capture of the problem or a sample document that triggers the fault. Additional attachments, if necessary, can be added after the bug is reported via Add a comment/attachment at the bottom of the page.

When you're done, click "Submit bug report".

If you require to collect data while off-line, u may use apport-cli for that. Detailed instructions on this, and many other tips can be found at ReportingBugs

The most current guide appears to be ReportingBugs - Community Ubuntu Documentation

It looks pretty up to date to me, and I think it would be redundant to put that information on here.

If you're having a X error (graphical subsystem), the X Debugging page is very useful:

• X/Debugging - Ubuntu Wiki

Usually doing an ubuntu-bug xorg will get all the right information to Launchpad. Bryce adds:

Yes, definitely a good recommendation. But also make sure to actually describe the problem. The most common mistake made in filing X bug reports is not fully describing what actually happened. Don't assume the logs alone tell the full story - be verbose.

If you know the application in question (specifically the package name), I would recommend using the ubuntu-bug command from a terminal. ubuntu-bug packagename will gather some automated information and take you right to the launchpad report a bug page, where you can describe the problem.

It's also a good time to search for potential duplicates, and if you find one click the "affects me too" button.

This help page has a lot more useful info: ReportingBugs - Community Ubuntu Documentation

It's as easy as visiting Launchpad and filling the 2 or 3 fields that are there. Command line is not the only way.

Alternatively, you can use this (replace PACKAGENAME in the URL with the package name. For example: unity, gnome-shell, etc):

http://bugs.launchpad.net/ubuntu/+source/PACKAGENAME/+filebug?no-redirect

It's mentioned in the Ubuntu documentation.

To report bugs to Debian use reportbug. Open Terminal and type:

$ reportbug

Is not installed by default. Install it with sudo apt install reportbug.

Now you can Report the Bug:

$ reportbug
Please enter the name of the package in which you have found a problem.
> packagename
*** Welcome to reportbug.  Use ? for help at prompts. ***
Detected character set: UTF-8
Please change your locale if this is incorrect.